Wind turbine anchor element

ABSTRACT

An anchor element for a tower includes an anchor plate element including an upper surface, a lower surface, and at least one first through-hole extending between the upper surface and the lower surface; and at least one first nut having a female thread, the at least one first nut being provided at the at least one first through-hole at the lower surface of the anchor plate element so as to cover the at least one first through-hole, wherein the at least one first nut is non-releasably connected to the lower surface of the anchor plate element.

BACKGROUND OF THE INVENTION

The subject matter described herein relates generally to methods and systems for anchoring a wind turbine tower, and more particularly, to methods and systems for anchoring a wind turbine tower to a tower foundation.

At least some known wind turbines include a tower and a nacelle mounted on the tower. A rotor is rotatably mounted to the nacelle and is coupled to a generator by a shaft. A plurality of blades extend from the rotor. The blades are oriented such that wind passing over the blades turns the rotor and rotates the shaft, thereby driving the generator to generate electricity.

As several other technical installations, a wind turbine requires a tower or a mast to which the machine nacelle and the wind rotor are mounted. Typically, the tower is made of steel and is connected to a foundation made of reinforced concrete. Typically, the connection is established by means of a flange with through-holes at the bottom of the tower. Anchor bolts are inserted into the through-holes and are fastened with nuts. Typically, the anchor bolts are connected to an anchor ring embedded in the foundation. Typically, the concrete surface of the foundation is relatively rough so that a grout joint is formed on which the flange is placed.

During operation of the wind turbine, varying wind loads are applied to the wind rotor and nacelle and are transferred to the foundation via the anchor bolts. Thus, it is desirable that damaged or broken bolts can be exchanged. Furthermore, it is desirable to inspect the anchor bolts so that damages can be detected at an early stage.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, an anchor element for a tower includes an anchor plate element comprising an upper surface, a lower surface, and at least one first through-hole extending between the upper surface and the lower surface; and at least one first nut having a female thread, said at least one first nut being provided at the at least one first through-hole at the lower surface of the anchor plate element so as to cover the at least one first through-hole, wherein said at least one first nut is non-releasably connected to the lower surface of the anchor plate element.

In another aspect, a wind turbine includes a tower having a lower portion; a foundation having an upper surface; an anchor including a lower anchor member within the foundation and comprising an upper surface, a lower surface, and a plurality of first through-holes connecting the upper and the lower surface of the lower anchor member, a plurality of lower nuts, each lower nut being provided at a respective first through-hole at the lower surface of the lower anchor member so as to cover said first through-hole and being non-releasably connected to the lower surface of the lower anchor member; an upper anchor member comprising an upper surface, a lower surface, and a plurality of second through-holes connecting the upper and the lower surface of the upper anchor member, each second through-hole being substantially vertically aligned with one of the first through-holes; a plurality of guide tubes, each guide tube connecting a pair of first and second through-holes; a plurality of anchor bolts, each anchor bolt being accommodated within one of the guide tubes, a lower portion of said anchor bolt being screwedly engaged with an inner thread of a corresponding lower nut and an upper portion of said anchor bolt protruding from the upper surface of the upper anchor member; and a plurality of upper nuts, each upper nut fixing an upper portion a corresponding anchor bolt; wherein the lower portion of the tower is rigidly connected to the upper surface of the upper anchor member.

In yet another aspect, a method for replacing an anchor bolt of a wind turbine includes loosening and removing an upper nut at an upper anchor member, removing the anchor bolt by unscrewing a lower portion of the anchor bolt from a lower nut which is non-releasably connected to a lower anchor member and pulling the anchor bolt out of a conduit extending between the lower anchor member and the upper anchor member, inserting an exchange anchor bolt into the conduit and screwing together a lower portion of the exchange anchor bolt and the lower nut, forming a rigid connection between the upper and lower anchor members by fastening the upper nut at an upper portion of the exchange anchor bolt.

Further aspects, advantages and features of the present invention are apparent from the dependent claims, the description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures wherein:

FIG. 1 is a perspective view of an exemplary wind turbine.

FIG. 2 is a view of an anchor comprising anchor elements according to embodiments described herein.

FIG. 3 is a cross-sectional detail view of an anchor element according to embodiments described herein.

FIGS. 4 and 5 show top views of anchor plate elements according to embodiments described herein.

FIGS. 6 and 7 show an anchoring device according to embodiments described herein.

FIGS. 8 to 10 illustrate different embodiments of a cap nut according to embodiments described herein.

FIG. 11 illustrates a method for replacing an anchor bolt according to embodiments described herein.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the various embodiments, one or more examples of which are illustrated in each figure. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet further embodiments. It is intended that the present disclosure includes such modifications and variations.

The embodiments described herein include a wind turbine system in which the anchor bolts can be easily exchanged and replaced. More specifically, damaged anchor bolts can be easily replaced. In addition, an inspection of the anchor bolts is enabled.

As modern wind turbines produce more and more power, they also have to bear higher loads. These loads are transferred to the foundation via the means for fastening the tower to the foundation including anchor bolts. High fatigue loads that are transferred via the anchor bolts may lead to an initial crack in the material and cause further propagation of this crack. Before or after breakage of the bolt, the bolt should be exchanged in order to maintain the structural integrity of the bolt connection. As anchor bolts are normally fixed inside a concrete foundation, it is typically an elaborate and costly task to exchange such an anchor bolt.

However, problems may occur when there forms a bond between the anchor bolt and the concrete. In other cases, a connection between a nut and the anchor bolt is stronger than the connection between the nut and the surrounding concrete. When trying to unscrew the anchor bolt from the nut, it may therefore occur that the connection between the fixing nut and the surrounding concrete loosens and breaks leaving the nut and the anchor bolt inseparably connected. Accordingly, the nut and anchor bolt turn loose below a lower anchor plate, thus rendering it impossible to unscrew the anchor bolt from the nut.

In view of the above, the present disclosure proposes to fix the position of the nut such that after an initial installation the nut is no longer able to move or turn. Also, it is suggested to cover or protect the anchor bolt from concrete or other materials. By these measures, the anchor bolt may be unscrewed from the nut and exchanged.

It is an advantage of the embodiments disclosed herein that, upon exchange of a damaged or broken anchor bolt, the nut fixing a lower portion of the damaged anchor bolt will not turn which allows removing the damaged anchor bolt without damaging the foundation. This avoids a partial or total rebuilding of the foundation.

It is another advantage of the embodiments disclosed herein that it is not necessary to leave damaged anchor bolts within the foundation as it was done in the past. This will improve tower stability and structural integrity.

It is yet another advantage of the embodiments disclosed herein that the disclosed methods of exchanging a damaged anchor bolt are cheaper than alternative solutions for fixing broken anchor bolts.

As used herein, the term “anchor element” is intended to be representative of a structure embedded or attached to a foundation, wherein the structure is designed and adapted so that a bottom part of wind turbine tower can be connected to it. As used herein, the term “blade” is intended to be representative of any device that provides a reactive force when in motion relative to a surrounding fluid. As used herein, the term “wind turbine” is intended to be representative of any device that generates rotational energy from wind energy, and more specifically, converts kinetic energy of wind into mechanical energy. As used herein, the term “wind generator” is intended to be representative of any wind turbine that generates electrical power from rotational energy generated from wind energy, and more specifically, converts mechanical energy converted from kinetic energy of wind to electrical power.

FIG. 1 is a perspective view of an exemplary wind turbine 10. In the exemplary embodiment, wind turbine 10 is a horizontal-axis wind turbine. Alternatively, wind turbine 10 may be a vertical-axis wind turbine. In the exemplary embodiment, wind turbine 10 includes a tower 12 that extends from a tower foundation 150, a nacelle 16 mounted on tower 12, and a rotor 18 that is coupled to nacelle 16. Rotor 18 includes a rotatable hub 20 and at least one rotor blade 22 coupled to and extending outward from hub 20. In the exemplary embodiment, rotor 18 has three rotor blades 22. In an alternative embodiment, rotor 18 includes more or less than three rotor blades 22. In the exemplary embodiment, tower 12 is fabricated from tubular steel to define a cavity (not shown in FIG. 1) between support system 14 and nacelle 16. In an alternative embodiment, tower 12 is any suitable type of tower having any suitable height.

Rotor blades 22 are spaced about hub 20 to facilitate rotating rotor 18 to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. Rotor blades 22 are mated to hub 20 by coupling a blade root portion 24 to hub 20 at a plurality of load transfer regions 26. Load transfer regions 26 have a hub load transfer region and a blade load transfer region (both not shown in FIG. 1). Loads induced to rotor blades 22 are transferred to hub 20 via load transfer regions 26 and then down into tower foundation 150 via tower 12.

In one embodiment, rotor blades 22 have a length ranging from about 15 meters (m) to about 91 m. Alternatively, rotor blades 22 may have any suitable length that enables wind turbine 10 to function as described herein. For example, other non-limiting examples of blade lengths include 10 m or less, 20 m, 37 m, or a length that is greater than 91 m. As wind strikes rotor blades 22 from a direction 28, rotor 18 is rotated about an axis of rotation 30. As rotor blades 22 are rotated and subjected to centrifugal forces, rotor blades 22 are also subjected to various forces and moments. As such, rotor blades 22 may deflect and/or rotate from a neutral, or non-deflected, position to a deflected position.

FIG. 2 is a view of an anchor 200 which is placed within a tower foundation 150 which, e.g., may be a foundation of a wind turbine 100. The anchor includes a lower anchor member 217 and an upper anchor member 315, the latter being a part of a tower base member 300. The lower anchor member 217 and the upper anchor member 315 are connected to each other by anchor bolts 340. The anchor bolts 220 are screwed together with lower nuts 225 which are mechanically fixed at a lower surface of a lower anchor plate. At the upper end, the anchor bolts 340 are fixed to the upper anchor member 315 by upper nuts 325. The anchor bolts 340 are accommodated in guide tubes (not shown in FIG. 2) extending between the lower surface of the upper anchor member and the upper surface of the lower anchor member. The guide tubes protect the anchor bolts 340 from concrete of the foundation which may harm the connection between the lower nuts 325 and the anchor bolts 340.

The tower base member 300 and the upper anchor member 315 both have a cylindrically symmetric shape and are machined as one piece. According to some embodiments, both the lower anchor member 217 and the upper anchor member 315 are plates and have a shape of a closed ring. According to some embodiments, the ring-shaped members are formed from a plurality of sectoral pieces, in a non-limiting example eight to twelve sectoral pieces. The pieces may then be fixed to each other, for example by welding, at the construction site. Alternatively, the plate members may be formed in one piece.

FIG. 2 shows that the lower anchor member 217 is placed in a lower part of the foundation 150. The upper ends of anchor bolts 340 protrude from foundation 150 at a point where a recess is formed. The upper anchor member 315 is placed over the upper ends of anchor bolts 340 such that anchor bolts 340 protrude through second through-holes formed within upper anchor member 315. Here, the tower base member 300 is aligned such that the side walls of the tower base member 300 are vertical. Then, a grout joint 360 is produced within a gap between foundation 150 and the lower end of upper anchor member 315. After the grout joint 360 has cured, tower foundation 150 is finished including tightening anchor bolts 340 to upper anchor member 315 by means of upper nuts 325. Thereafter, the tower can be built further to the top.

According to another embodiment, upper anchor member 315 may be a part which is separate from tower base member 300. In that case, upper anchor member 315 may be connected to a lower tower portion by other means, for example by welding, screwing, riveting or using further anchor bolts.

FIG. 3 shows a detailed cross-sectional view of an anchor element 200 according to an embodiment. The anchor element 200 includes an anchor plate element 215, a tower base member 300, a conduit 350, an anchor bolt 340, a first nut 220, a second nut 320, a first washer 230, and a second washer 330. The tower base member 300 includes a tower base plate 310 and a T-flange 316 for connecting the anchor element 200 to a lower portion of tower 12. In a typical embodiment, anchor plate element 215 is a ring sector as shown in FIG. 4. In the embodiment shown in FIG. 4, the ring sector spans 40° degrees so that nine ring sector element make a full 360° ring. However, the 360° degree ring-shaped lower anchor member 217 may be formed from less or more than nine anchor elements. Accordingly, the ring sectors may span more or less than 40° degrees. Typically, the ring sectors are welded to each other before foundation 150 is filled with concrete.

The anchor element 200 shown in FIG. 3 includes an anchor plate element 215 having an upper surface 211, a lower surface 212 and a first through-hole 213 which extends between the upper surface 211 and the lower surface 212. The anchor plate element 215 may be assembled with other anchor plate elements to form a ring-shaped anchor plate 210. Typically, the upper 211 and lower surfaces 212 are flat surfaces and parallel to each other. Typically, the first through-hole 213 is a cylindrical aperture having a constant diameter over its length and is formed perpendicular to surfaces 211 and 212.

The anchor element 200 further includes a tower base plate 310 provided above the anchor plate element 215 or the anchor plate 210. The tower base plate 310 has an upper surface 311, a lower surface 312, and a second through-hole 313 which extends between the upper surface 311 and the lower surface 312. Typically, tower base plate 310 has a cylindrically symmetric shape and is machined as one piece. However, it may be formed from several pieces in a similar way as anchor plate 210. Tower base plate 310 forms a part of tower base member 300 which is the lowest part of the tower 12. The tower base plate 310 further includes a T-flange 316 which is used to connect tower 12 to the tower base member 300.

The upper 311 and lower surfaces 312 are flat surfaces and parallel to each other. Each second through-hole 313 is a cylindrical tube having a constant diameter over its length and is perpendicular to surfaces 311 and 312. The diameter of the second through-hole 313 may be equal or different than the diameter of the first through-hole 213. Furthermore, the second through-hole 313 is vertically aligned with the first through-hole 213.

Tower base plate 310 and anchor plate 210 are connected to each other by anchor bolt 340, a first washer 230, a second washer 330, first nut 220, and second nut 320. Within the completed foundation 150, the anchor is surrounded by concrete. However, the concrete does not come into contact with anchor bolt 340 since the conduit 350 and the first nut 220 shield anchor bolt 340 from the concrete.

In one embodiment, conduit 350 is a cylindrical tube which has a diameter slightly smaller than first through-hole 213 and second through-hole 313. Conduit 350 has a length so that it may be disposed between the upper surface 311 of the tower base plate 310 and the lower surface 212 of the anchor plate element 215 within first through-hole 213 and second through-hole 313. According to an alternative embodiment, conduit 350 may have a slightly larger diameter than the first 213 or second through-hole 313 and may be attached to upper surface 211 or lower surface 312 around a respective through-hole 213, 313.

According to the embodiment of FIG. 3, the anchor element 200 further includes anchor bolt 340 which is accommodated within conduit 350 and fixed below a lower surface 212 of the anchor plate 210 and above an upper surface 311 of the tower base plate 310. Anchor bolt 340 includes a lower anchor bolt external thread 341 at a lower portion of the anchor bolt 340 and an upper anchor bolt external thread 342 at an upper portion of the anchor bolt 340.

The lower anchor bolt external thread 341 is screwed together with the internal thread of first nut 220. The upper portion of anchor bolt 340 protrudes from the upper surface 311 of the tower base plate 310 where upper anchor bolt external thread 342 is screwed together with the internal thread of second nut 320. The second washer 330 is disposed between second nut 320 and upper surface 311 of the tower base plate 310.

The first nut 220 is provided around the rim of first through-hole 213 at the lower surface 212 of the anchor plate element 215. First nut 220 is connected in a non-releasable manner to the lower surface 212 of anchor plate 210. For example, first nut 220 may be welded to lower surface 212 or be integrally formed with lower surface 212 in a different way. Typically, first nut 220 is a closed cap nut but may also be an open cap nut according to another embodiment.

According to the embodiment of FIG. 3, a first washer 230 is provided between lower surface 212 of anchor plate element 215 and first nut 220. According to this embodiment, first washer 230 is spot-welded to lower surface 212 and first nut 220 is spot-welded to first washer 230. According to another embodiment, a stiff connection between anchor plate element 215 and first nut 220 is established mechanically.

According to yet another embodiment, first nut 220 is closed at an end distal from anchor plate element 215. Thus, it may be avoided that concrete flows inside first nut 220 and does harm to the connection between anchor bolt 340 and first nut 220.

To provide a better grip of first nut 220 to the concrete and, thus, to avoid turning loose of first nut 220 and anchor bolt 340, a torque member 221 is provided at nut 220. Torque member 221 is adapted for supporting the cap nut in the concrete. If the screw joint between bolt 340 and nut 220 is loosened, torque member 221 absorbs the applied torque and provides sufficient counter torque so that cap nut 220 is held in place. Thus, unscrewing bolt 340 from cap nut 220 is enabled even if the connection between cap nut 220 and lower surface 212 breaks.

In one embodiment, torque member 221 is a metal plate having essentially the same thickness as the wall of the cap nut. A central area of the torque member 221 is fixed to the cap nut at a portion of the convex cap of the cap nut, typically at the apex thereof. According to another embodiment, the torque member is a plate provided at a cylindrical outer surface of the cap nut and substantially extending along an axial direction of the nut. Yet further embodiments for the torque member 221 are described below with reference to with FIGS. 6 and 7.

FIGS. 4 and 5 show top views of an anchor plate element 215. FIG. 4 illustrates an anchor plate element 215 in the shape of a ring sector spanning 40° degrees or one-ninth of a full circle. Accordingly, nine anchor plate elements 215 like the one depicted in FIG. 4 may be used to form a full ring, i.e. to form anchor plate 210. Typically, the nine anchor plate elements 215 are assembled by spot-welding the anchor plate elements 215 to each other.

Anchor plate element 215 of FIG. 4 has two rows of through-holes 213. According to a typical embodiment, the number of through-holes 213 in a full ring is 54 or 62 for a single row. Therefore, the anchor plate element 215 of FIG. 4 includes only one-ninth, i.e. six through-holes in each row. It will be understood that these numbers are merely examples and may be higher or lower in actual embodiments of the present invention.

FIG. 5 illustrates an anchor plate element 215 having the shape of a full ring, i.e. spanning 360° degrees. As explained with reference to FIG. 4, each row of the anchor plate according to this embodiment includes 54 through-holes, resulting in a total of 108 through-holes for the whole anchor plate element.

FIGS. 6 and 7 show different views of an embodiment of a torque member 221. FIG. 6 shows a cross sectional side view of the first nut 220 with torque member 221. FIG. 7 shows a top view of the same first nut 220 depicted in FIG. 6. Torque member 221 is a wing shaped metal plate extending in a direction substantially parallel to the longitudinal axis of cap nut 220. Thus, the wing-shaped torque member 221 will resist any torque applied to cap nut 220 when anchor bolt 340 is screwed together or unscrewed therefrom.

FIGS. 8 to 10 illustrate different realizations of cap nut 220. In the embodiments shown in FIGS. 8 to 10, a lower anchor bolt external thread 341 of anchor bolt 340 forms a screw joint with an internal thread (not shown in FIGS. 8 to 10) of cap nut 220. Furthermore, first nut 220 is non-releasably connected to a lower surface 212 of an anchor plate 215 element in the embodiments shown in FIGS. 8 to 10.

FIG. 8 shows the lower part of anchor element 200 in which first nut 220 is spot-welded to lower surface 212 of anchor plate member 215 around the rim of first through-hole 213.

FIG. 9 shows a different embodiment of first nut 220 in which first nut 220 includes a washer which is integrally formed with first nut 220. According to this embodiment, first nut 220 is spot-welded to lower surface 212 of anchor plate member 215 around the rim of first through-hole 213.

FIG. 10 shows yet another embodiment of anchor plate member 215 in which first nut 220 is integrally formed with the lower surface 212 of anchor plate element 215. Due to the omission of welding, the long-term stability of the connection of first nut 220 to anchor plate member 215 is improved.

FIG. 11 illustrates a method 400 for replacing an anchor bolt. After the start of the method, it is decided in a first block 405 whether there is a broken or damaged anchor bolt. In case there is no broken or damaged anchor bolt, the method terminates. In case there is at least one broken or damaged anchor bolt 340, the method continues to block 410.

In the next block 410, a second nut 320 securing a connection between an anchor bolt 340 and an upper surface 311 of a tower base plate 310 is loosened and removed. In this context, the tower base plate 310 is considered a part of a wind turbine tower base member 300.

In a next block 415, the anchor bolt 340 is removed by unscrewing the anchor bolt 340 from of a first nut 220 which is connected to an anchor plate element 215, and guided out of a conduit 350 extending between the anchor plate element 215 and the tower base plate 310.

In the next block 420, an exchange anchor bolt is inserted into the conduit 350 and screwedly fixed to the first nut 220.

In the following blocks 425, 430 and 435 the second nut 320 is fastened onto the exchange anchor bolt 340 on an upper surface 311 of the tower base plate 310.

To this end, in block 425 the exchange anchor bolt 340 is pulled into a direction orthogonal to the surface of the tower base plate 310 such that a gap is formed between said second nut 320 and the tower base plate 310, thus prestressing the anchor bolt.

In the next block 435, the second nut 320 is fastened such that the gap vanishes. In the following block 440 the pulling force is released such that the anchor bolt 340 is pressed towards the tower base plate 310. This fastens the second nut 320 onto the exchange anchor bolt 340 and leaves bolt 340 prestressed. According to some embodiments, the second nut 320 may be tightened on the exchange anchor bolt 340 with a predetermined torque.

In the next block 440 it is decided whether there still is a broken or damaged anchor bolt 340. In case there is still at least one broken or damaged anchor bolt 340, the method goes back to step 410. In case there is no broken or damaged anchor bolt 340, the method terminates. Alternatively, the method may terminate after block 435, i.e. after exchanging a single anchor bolt.

Exemplary embodiments of anchor elements and methods for exchanging anchor bolts are described above in detail. The anchor elements and methods for exchanging anchor bolts are not limited to the specific embodiments described herein, but rather, components of the elements and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the anchor elements may also be used for antenna towers or bridge pylons, and are not limited to practice with only the wind turbine systems as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other applications.

Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. While various specific embodiments have been disclosed in the foregoing, those skilled in the art will recognize that the spirit and scope of the claims allows for equally effective modifications. Especially, mutually non-exclusive features of the embodiments described above may be combined with each other. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 

1. An anchor element for a tower, comprising: an anchor plate element comprising an upper surface, a lower surface, and at least one first through-hole extending between the upper surface and the lower surface; and at least one first nut having a female thread, said at least one first nut being provided at the at least one first through-hole at the lower surface of the anchor plate element so as to cover the at least one first through-hole, wherein said at least one first nut is non-releasably connected to the lower surface of the anchor plate element.
 2. The anchor element according to claim 1, wherein the at least one first nut is connected to the lower surface of the anchor plate element such that a lower portion of the at least one first through-hole is sealed from the space below the lower surface of the anchor plate element.
 3. The anchor element according to claim 1, wherein a shape of the anchor plate element is an ring sector.
 4. The anchor element according to claim 1, wherein a shape of the anchor plate element is a closed ring.
 5. The anchor element according to claim 1, wherein the at least one first nut is closed at the end distal from the anchor plate element.
 6. The anchor element according to claim 1, wherein the at least one first nut is a cap nut.
 7. The anchor element according to claim 6, wherein the cap nut further comprises a torque member adapted for anchoring the cap nut in concrete when the anchor element is embedded within a tower foundation.
 8. The anchor element according to claim 7, wherein a cap of the cap nut has a convex shape and the torque member comprises a plate which is tangent to the cap of the cap nut.
 9. The anchor element according to claim 7, wherein the torque member comprises a plate attached to an outer surface of the cap nut and extending at least partially in an axial direction of the cap nut.
 10. The anchor element according to claim 1, wherein the at least one first nut is integrally formed with the lower surface of the anchor plate element.
 11. The anchor element according to claim 1, wherein at least one first washer is provided between the lower surface of the anchor plate element and the at least one first nut, the at least one first washer being non-releasably connected to both the lower surface of the anchor plate element and the at least one first nut.
 12. The anchor element according to claim 1, wherein the at least one first nut is integrally formed with a washer.
 13. The anchor element according to claim 1, further comprising: a tower base plate provided above the anchor element, the tower base plate having an upper surface, a lower surface, and at least one second through-hole extending between the upper surface of the tower base plate and the lower surface of the tower base plate; at least one conduit extending between the lower surface of the tower base plate and the upper surface of the anchor plate element and connecting said at least one first and at least one second through holes; and at least one anchor bolt accommodated within the at least one conduit, a lower portion of the anchor bolt being screwedly fixed to the female thread of the at least one first nut and an upper portion of the at least one anchor bolt protruding from the upper surface of the tower base plate and being fixed to a second nut provided at the upper surface of the tower base plate.
 14. A wind turbine comprising: a tower having a lower portion; a foundation having an upper surface; an anchor comprising: a lower anchor member within the foundation and comprising an upper surface, a lower surface, and a plurality of first through-holes connecting the upper and the lower surface of the lower anchor member, a plurality of lower nuts, each lower nut being provided at a respective first through-hole at the lower surface of the lower anchor member so as to cover said first through-hole and being non-releasably connected to the lower surface of the lower anchor member; an upper anchor member comprising an upper surface, a lower surface, and a plurality of second through-holes connecting the upper and the lower surface of the upper anchor member, each second through-hole being substantially vertically aligned with one of the first through-holes; a plurality of guide tubes, each guide tube connecting a pair of first and second through-holes; a plurality of anchor bolts, each anchor bolt being accommodated within one of the guide tubes, a lower portion of said anchor bolt being screwedly engaged with an inner thread of a corresponding lower nut and an upper portion of said anchor bolt protruding from the upper surface of the upper anchor member; and a plurality of upper nuts, each upper nut fixing an upper portion a corresponding anchor bolt; wherein the lower portion of the tower is rigidly connected to the upper surface of the upper anchor member.
 15. The wind turbine according to claim 14, further comprising: a plurality of first washers, each first washer being provided between the lower surface of the lower anchor member and one of the lower nuts and being non-releasably connected to both the lower surface of the lower anchor member and said lower nut.
 16. The wind turbine according to claim 14, wherein at least one of the lower anchor member and the upper anchor member is a plate having a ring shape.
 17. The wind turbine according to claim 14, wherein said lower nuts are cap nuts.
 18. A method for replacing an anchor bolt of a wind turbine, comprising: (a) loosening and removing an upper nut at an upper anchor member, (b) removing said anchor bolt by unscrewing a lower portion of said anchor bolt from a lower nut which is non-releasably connected to a lower anchor member and pulling said anchor bolt out of a conduit extending between the lower anchor member and the upper anchor member, (c) inserting an exchange anchor bolt into said conduit and screwing together a lower portion of said exchange anchor bolt and said lower nut, (d) forming a rigid connection between the upper and lower anchor members by fastening said upper nut at an upper portion of said exchange anchor bolt.
 19. The method according to claim 18, wherein the rigid connection is formed by prestressing the anchor bolt.
 20. The method according to claim 19, wherein prestressing the bolt includes pulling said exchange anchor bolt into an axial direction thereof such that a gap is formed between the upper nut and the upper anchor member; tightening the upper nut; and releasing the pulling force. 